The invention involves a procedure for flow measurement in which a marker volume is generated by the application of electric voltage, this marker volume running through a measured length in the direction of flown the flow speed or a measurement linked to the flow speed being calculated from the transit time of the marker volume.
A number of technical measuring procedures are known for measuring the through-flow of flowing media. A procedure for the flow measurement of non-conducting media is described in DE-A 29 44 979, in which the flowing medium is marked in pulses by ionisation and the flow can be calculated from the time taken by the marker to pass through a measured length.
An alternative proposal in the same published patent application is to heat the flowing medium in pulses with an appropriately charged heating wire and to determine the migration of the marker volume from the elevated temperature by means of thermistors or thermocouples.
Furthermore, ionisation flow meters, such as those described in U.S. Pat. Nos. 4,393,719 and 4,186,601 are also state of the art. A high- voltage source is used to generate pulsed ionisation of the flowing non-conducting medium, for example a fuel and air mixture for combustion engines, and the migration of the ionisation cloud in a measured length is determined.
The saline solution procedure according to van Allen's method is also known from the state of the art (cf. v. Allen. Ch M, "The salt velocity method of water measurement" Trans-ASME Vol 45 (1923), p. 285 and Calame, H "Impfverfahren mit chemischen und radioaktiven Substanzen, thermisches und kalorisches Me.beta.verfahren" (Inoculation method with chemical and radioactive substances, thermal and caloric measurement procedure), VDI Reports No 86 (1964), page 59). In this method, shots of a salt solution with a specific minimum concentration are injected into the flowing medium over a short period. Two pairs of electrodes are used to determine the change in conductivity of the flowing medium caused by the incorporated cloud of solution, with the aid of oscillographic transit-time measurement. The transit time diagram is evaluated from an integral mean of the change in conductivity. The measuring accuracy of the procedure is relatively high at 1% if the measurement mean is taken.
However, those methods of flow measurement previously reported cannot be used or can be used with only an unsatisfactory result if a flow measurement is to be performed on aqueous body-related fluids. The term "body-related fluids" is intended to include both infusion solutions and drug solutions, as well as dialysis solutions and body fluids, particularly the blood fluids exchanged with dialysis solutions. Body-related fluids also include fluids drawn off from or eliminated by the body. In general, the term body-related fluids can be used with reference to aqueous endogenous fluids or fluids passed into the body for nutrition or therapy these fluids having to flow or be passed to the body at speeds typical of those in the body itself.
Although the flow velocity of these body-related fluids is generally relatively low, in the order of magnitude of 10.sup.-3 cm/second, there are large differences in the flow speeds to be measured because of the broad range of variation. Thus, for example, a minimum value of 0.1 ml per hour is required for infusion solutions while the maximum value is 1 l per hour. This is a difference of a factor of 10.sup.4.